The present invention relates generally to variable speed drives. More specifically, the present invention relates to a variable speed drive that has multiple inverter outputs that can each power a corresponding load.
Many water chiller or refrigeration applications use multiple refrigeration circuits, i.e., two or more refrigeration circuits, each having one or more compressors dedicated to the refrigeration circuit. One purpose of the multiple or redundant refrigerant circuits and compressors is to provide improved reliability of the overall system by having one or more refrigerant circuits and compressors remain operational to provide a reduced level of cooling capacity in the event that a refrigerant circuit and/or compressor fails and can no longer provide cooling capacity.
The corresponding compressor motor for each compressor of a refrigeration circuit can be connected to the AC power grid at the system location. The connection of each compressor motor to the power grid permits the remaining refrigerant circuits and compressors to remain operational even if one refrigerant circuit and/or compressor has a failure. A drawback to connecting the compressor motors to the power grid is that all of the motors are provided only one input voltage and frequency, and thus, can generate only one output speed.
To operate a motor at more than one output speed, a variable speed drive can be inserted between the system power grid and the motor to provide the motor with power at a variable frequency and variable voltage. In the multiple circuit refrigeration system, variable speed operation of the motors can be obtained by providing a corresponding variable speed drive for each compressor motor or by connecting all of the compressor motors in parallel to the inverter output of a variable speed drive. One drawback of using a variable speed drive for each compressor is that the overall chiller system becomes more expensive because multiple drives with a given cumulative power rating are more expensive than a single drive of the same output power rating. One drawback to connecting the compressor motors in parallel to the single inverter output of the variable speed drive is that a fault or failure of one of the motors may disable the variable speed drive and thus prevent the other motors connected to the variable speed drive from operating the remaining compressors on the chiller system. This disabling of the other motors connected to the variable speed drive defeats the function of the redundant refrigerant circuits because all the refrigerant circuits are disabled as a result of the disabling of the motors and the variable speed drive.
Therefore, what is needed is a variable speed drive that can provide substantially independent power to multiple loads or motors in a multiple circuit refrigeration system.